Phenolic resole resins are prepared by the reaction of a phenolic compound with formaldehyde in the presence of an alkaline catalyst. Curing of the resole to a crosslinked mass is accelerated by addition of an acid which also serves to neutralize, in whole or in part, the alkaline catalyst residue.
This curing is extremely important in that it should preferably be very fast and very complete so as to shorten the processing time required.
Processing speed is important in many industrial resole resin applications for cost considerations and energy conservation. In making paper filters for example, the substrate paper is drawn through a dip tank containing a phenolformaldehyde resole resin binder solution, then drawn continously through a heated oven to remove most of the volatile material and advance the resin somewhat. The faster the rate of cure, the more energy efficient is the process.
Several ways to improve processing speed are known. One is to increase the molecular weight of the resin and the other is to use higher solids resin solutions. Both methods suffer because of problems in getting adequate penetration of the resin into the substrate, leading to poorer final laminate appearance and greater water absorption which also gives poorer electrical properties. Generally, a low viscosity resin solution and/or low molecular weight resin is desirable for good substrate penetration and the molecular weight of the resin is raised during the heating step. Increasing the molecular weight in the impregnated substrate, (the prepreg), is needed so that when the prepreg is cured under heat and pressure, excessive resin flow out of the substrate is not encountered.
Another method of improving the cure rate and therefore the processing speed is the selection of a suitable acid catalyst. It is this preferred technique that is the focus of the present invention which describes an accelerated resole composition comprising a neutralizing acid that gives unexpectedly advantageous cure rates.
The resole resin compositions of the present invention are particularly useful as components of binders in the production of fiber-glass insulation materials. Conventionally such products are made by forming a mat of glass fibers, spraying the mat with a binder and then curing the binder. The cured binder serves to stabilize the glass fiber structure dimensionally such that it can be handled and installed easily. One problem with such insulation sheet materials is their bulk which is such that it is highly attractive, from an economic standpoint, to compress them to a fraction of their original thickness before transporting them. It is therefore necessary that the resin-bonded glass fiber mat recover as much as possible of its original thickness when the compressive forces are released. This property is known as the "Recovery" of the mat. Recovery has been no great problem using glass mats prepared from borosilicate glass fibers. However with a high soda or soda-lime glass it has been found that conventional resole compositions do not function so well. Recoveries of over 80 percent, are relatively easily obtained with borosilicate glass, but recoveries of only around 75 percent are typical using existing resole compositions with the soda-lime type of glass.
One particular advantage of the resole compositions of the present invention is that they are capable of giving a glass fiber insulation material with excellent recovery after compression even with soda-lime glass. The "recovery" obtained will depend on the mat employed and the amount of resin used as well as the conditions. The results are therefore essentially comparative within a series run with only one variable changed. Generally the test employs a glass fiber mat sprayed with a fixed amount of the resin, usually from 10-15% weight of the treated mat weight which is then compressed to a given fraction, say about to 25 percent of its original thickness for 2 hours at 95 percent humidity and 68.degree. C. before being released and conditioned under standard temperature and humidity conditions.